Commit cee0f082df0bb0f90c7ee38c98ba8071c3be8d0c | zig

author	Andrew Kelley <andrew@ziglang.org> 2022-01-31 22:59:49-05:00
committer	Andrew Kelley <andrew@ziglang.org> 2022-02-02 22:07:56-07:00
log	cee0f082df0bb0f90c7ee38c98ba8071c3be8d0c
tree	899099c7f51b7c055ee3c8c5fdd906ee0b915eb4
parent	23d148e5c7ab036ef686606f11a2343940890f5a

std: correct rounding in parse_hex_float.zig

1 files changed, 11 insertions(+), 9 deletions(-)

lib/std/fmt/parse_hex_float.zig+11-9

 @@ -202,18 +202,19 @@ pub fn parseHexFloat(comptime T: type, s: []const u8) !T {
         exponent += 1;
     }
-    // There are two cases to handle:
+    // Whenever the guard bit is one (G=1) and:
-    // - We've truncated more than 0.5ULP (R=S=1), increase the mantissa.
+    //   - we've truncated more than 0.5ULP (R=S=1)
-    // - We've truncated exactly 0.5ULP (R=1 S=0), increase the mantissa if the
+    //   - we've truncated exactly 0.5ULP (R=1 S=0)
-    //   result is odd (G=1).
+    // Were are going to increase the mantissa (round up)
-    // The two checks can be neatly folded as follows.
+    const guard_bit_and_half_or_more = (mantissa & 0b110) == 0b110;
-    mantissa |= @boolToInt(mantissa & 0b100 != 0);
-    mantissa += 1;
     mantissa >>= 2;
     exponent += 2;
-    if (mantissa & (1 << (mantissa_bits + 1)) != 0) {
+    if (guard_bit_and_half_or_more) {
+        mantissa += 1;
+    }
+    if (mantissa == (1 << (mantissa_bits + 1))) {
         // Renormalize, if the exponent overflows we'll catch that below.
         mantissa >>= 1;
         exponent += 1;
 @@ -338,6 +339,7 @@ test "f128" {
         // // Min denormalized value.
         .{ .s = "0x1p-16494", .v = math.f128_true_min },
         .{ .s = "-0x1p-16494", .v = -math.f128_true_min },
+        .{ .s = "0x1.edcb34a235253948765432134674fp-1", .v = 0x1.edcb34a235253948765432134674fp-1 },
     };
     for (cases) |case| {